FIG. 1 shows a constitution of a conventional optical sensing circuit used for a pointing device. A circuit XCT 100 is provided for producing an X signal indicating a moving amount in an X direction and a moving direction, and a circuit YCT 100 is provided for producing a Y signal indicating a moving amount in a Y direction and a moving direction.
Between a power supply voltage VCC terminal and a ground voltage VSS terminal, a resistor RLED, a light emitting diode (LED) XLED for producing the X signal contained in the circuit XCT 100, and a LED YLED for producing the Y signal contained in the circuit YCT 100 are connected in series in order to reduce the amount of current.
In the circuit XCT 100, two signal producing paths are set up in parallel as a circuit of a photo receiving side. As a first path, a phototransistor X1PT and a resistor X1R are connected in series between the power supply voltage VCC terminal and the ground voltage VSS terminal, and a node X1 between the phototransistor X1PT and the resistor X1R is connected to one input terminal of a comparator X1COMP.
As a second path, a phototransistor X2PT and a resistor X2R are connected in series between the power supply voltage VCC terminal and the ground voltage VSS terminal, and a node X2 between the phototransistor X2PT and the resistor X2R is connected to one input terminal of a comparator X2COMP. Reference voltage Vref is applied to the other input terminal of each of the comparators X1COMP, X2COMP.
A rotary slit XSLT is arranged between the LED XLED and the phototransistors X1PT, X2PT. This rotary slit XSLT is rotated in accordance with movement of the pointing device in an X direction, and transmits light emitted from the LED XLED to the phototransistors X1PT, X2PT, or interrupts it. Here, in the phototransistors X1PT, X2PT, current flow is varied in accordance with the amount of received light, and voltage at the nodes X1, X2 is accordingly varied. The phototransistor X1PT is oriented at predetermined angle relative to the phototransistor X2PT, and voltage waveforms at the nodes X1, X2 have an about 90 degrees phase difference from each other.
Because of the foregoing constitution, the circuit XCT 100 operates as follows. When the pointing device moves in the X direction, the rotary slit XSLT is rotated in accordance with a moving amount and a moving direction thereof, and the amounts of light received at the phototransistors X1PT, X2PT are varied, and currents flowing in X1PT, X2PT are also varied. These variations of currents are converted into voltages by the resistors X1R, X2R, extracted as voltage signals from the nodes X1, X2, and applied to the comparators X1COM, X2COM, respectively.
In the comparator X1COMP, the voltage V (X1) at the node X1 is compared with the reference voltage Vref. A low level voltage is outputted when the voltage V (X1) is below the reference voltage Vref, and a high level voltage is outputted when it is not less than the reference voltage Vref. Similarly, in the comparator X2COMP, the voltage V (X2) at the node X2 is compared with the reference voltage Vref. A low level voltage is outputted when the voltage V (X2) is below the reference voltage Vref, and a high level voltage is outputted when it is not less than the reference voltage Vref. Thus, the rotation of the rotary slit XSLT, that is, how far the pointing device moves in the X direction, is detected with the pulse output from the comparator X1COMP. Additionally, because of the phase difference between the signals X1, X2 as described above, a moving direction can also be detected.
The circuit YCT 100 also has a constitution for receiving light from the LED YLED similar to that of the circuit XCT 100. Specifically, the circuit YCT 100 comprises the rotary slit YSLT, phototransistors Y1PT, Y2PT, resistors Y1R, Y2R, and comparators Y1COMP, Y2COMP, and operates similar to the circuit XCT 100. Thus, explanation thereof will be omitted.
However, the following problems have been inherent in such a conventional optical sensing circuit.
FIG. 2A shows respective voltages V (X1), V (X2) at the nodes X1, X2. Further, FIG. 2B shows output waveforms of the comparators X1COMP, X2COMP when a threshold (=reference voltage Vref) of the comparators X1COMP, X2COMP is Vth1 shown in FIG. 2A. FIG. 2C shows output waveforms of the comparators X1COMP, X2COMP when a threshold of the comparators X1COMP, X2COMP is Vth2 shown in FIG. 2A.
To identify a rotational direction of the rotary slit XSLT, the threshold voltage Vth must be in a range between upper and lower points C1, C2 at which the waveforms of the voltages V (X1), V (X2) at the nodes X1, X2 intersect each other.
As the threshold Vth1 ranges between the points C1, C2, for the outputs of the comparators X1COMP, X2COMP, there are an overlapping period 10a of high levels and an overlapping period 10b of low levels as shown in FIG. 2B. In such a case, it is possible to identify the rotational direction of the rotary slit XSLT. For example, in the period 10b where both outputs are low, the output of the comparator X1COMP first rises to a high level, whereby the rotational direction can be detected.
However, if the threshold voltage Vth is at the intersection point C1 as in the case of a threshold Vth2, or above the point C1, as shown in FIG. 2C, there is an overlapping period 12b of low levels while there is no overlapping period 12a of high levels. In such a case, it is impossible to identify the rotational direction of the rotary slit XSLT.
If output characteristics or light intensity of the LED is higher than those shown in FIG. 2A, or sensitivity of the phototransistor is higher, the voltages V (X1), V (X2) at the nodes X1, X2 respectively become similar to those shown in FIG. 3A. FIG. 3B shows respective output waveforms of the comparators X1COMP, X2COMP when threshold of the comparators X1COMP, X2COMP is Vth3 shown in FIG. 3A in this case. FIG. 3C shows respective output waveforms of the comparators X1COMP, X2COMP when threshold of the comparators X1COMP, X2COMP is Vth4 shown in FIG. 3A.
As the threshold Vth3 ranges between points C3, C4 at which the waveforms of the voltages V (X1), V (X2) intersect each other, for outputs of the comparators X1COM, X2COMP, as shown in FIG. 2B, there are an overlapping period 20a of high levels and an overlapping period 20b of low levels. Thus, it is possible to identify the rotational direction of the rotary slit XSLT.
However, if the threshold voltage Vth is at the point C4 of waveform intersection as in the case of a threshold Vth4, or below the point C4, as shown in FIG. 3C, there is an overlapping period 22a of high levels, while there is no overlapping period 22b of low levels. Also in such a case, it is impossible to identify the rotational direction of the rotary slit XSLT.
Normally, the LED or the phototransistor used for the pointing device greatly varies in light intensity or receiving sensitivity even under the same conditions. Accordingly, the respective elements are classified into several ranks and, in accordance with the rank, a value of the resistor RLED or values of the resistors X1R, X2R are adjusted for a normal operation.
However, there is still some variation even among the elements classified into the same rank. Therefore, the distance between the LED and the rotary slit or between the phototransistor and the rotary slit must be adjusted at the end.
Accordingly, if the light intensity emitted from the LED or the receiving sensitivity of the phototransistor is low as shown in FIG. 2A, or if the light intensity emitted from the LED or the receiving sensitivity of the phototransistor is high as shown in FIG. 3A, it may be difficult to set the threshold of the comparators within the range between the upper and lower points at which the waveforms of the output voltages V (X1), V (X2) of the phototransistors intersect each other.
Additionally, if the light intensity emitted from the LED or the receiving sensitivity of the phototransistor is high, as shown in FIG. 3A, the minimum voltage level of the waveforms of the voltages V (X1), V (X2) are considerably higher than the ground voltage VSS. For this reason, the case in which the light emitted from the LED is not interrupted by the rotary slit completely and thus received by the phototransistor, or light reflected on a portion other than the rotary slit is received by the phototransistor, or the like may often occur. If measures taken to counter such a phenomenon depend on mechanical structures or arrangements of the LED, the rotary slit and the phototransistors, the cost of the pointing device itself may be increased.